System for amplifying modulated waves



De@ 16, 1941. K. Pos-'rl-IUMUS;A 2,266,073 lSYSTEM AMPLIFYING MODULATEDWAVES Filed Jau'l. 5, 1940 3 SI-leets-Sheet 2 INVENTOR KLA/15 PUSTHUMUS7% MWL ATTORNEY Patented Dec. 16, .1941

SYSTEM FOR AMPLIFYING MODULATED WAVES Y y Klaas Posthumus, Eindhoven,

Netherlanus, 'assignor to Radio YCorporation of America, New

York, N.` Y.

Application January 5, 1940, Serial No.

In the Netherlands November 10, 1938 8 Claims.

This invention relates to a system for amplifying modulatedoscillations, which is particularly suitable for the final stage of atransmitter modulated in a preceding stage. The arrangement of thesystem may also be such that the modulation takes place in the finalstage itself.

In order to improve the efiiciency of the final stage of a transmitter,it has previously been proposed to compose the amplifier of acombination of a class B and a class C amplifier. With an unmodulatedcarrier-wave amplitude the class B amplifier is fully loaded, so thatthe alternating voltage set up by this amplifier can no longer increase;but the current may still grow. When the instantaneous value of themodulated carrier wave increases so as to exceed the unmodulated carrierwave amplitude, the class C amplifier starts operating and suppliesenergy to a common load impedance. Between the class B or the class Camplifier and the common load impedance is connected a so-calledimpedance inverting network which possesses the property that theimpedance occurring between the input terminals is inverselyproportional to the impedance connected between the output terminals,and inversely. Y

Applicant proposed in United States application #168,155, filed October9, 1937, now Patent No. 2,230,122, January 28, 1941, to utilize a classB amplifier and three class C amplifiers, which are connected in asuitable manner through the intermediary of impedance inverting networksto a common load impedance. The circuit arrangement is such, that withan instantaneous value of the high-frequency amplitude which is lowerthan the umnodulated carrier wave amplitude,

the class B amplifier is fully loaded. At this instantaneous value oneof the class C amplifiers starts operating owingto the fact that thisamplifier has a threshold value which is equal to the said instantaneousvalue of the carrier wave amplitude. These conditions are maintaineduntil at a sufficiently large modulation depth, the instantaneous valueof the highfrequency amplitude surpasses the threshold value of the twoother class C amplifiers, whose threshold values are mutually equal,whereupon the class C amplifiers also supply energy to the loadimpedance. With a 100% modulation depth all the amplifiers supply inthis case the same amount of energy to the load impedance.

In describing my invention, reference will be made to the attacheddrawings, wherein Figures 1 and 2 show diagrammatically efficientamplifying systems of the type involved here and such as disclosed in mysaid prior application. These figures are used herein to explain thenature of the systems involved and the object of my invention which isto provide improved amplifiers of this type; Figures 3 and 4 showdiagrammatically amplifying systems of the present invention comprisingseveral stages and their couplings and the coupling of the amplifier toa load impedance; Figures 5 and 7 illustrate somewhatin greater detailefiicient amplifying systems of the nature shown diagrammatically inFig. 3; while Figure 6 illustrates in somewhatgreater detail anefficient amplifying system of the nature shown diagrammatically inFigure 4 of the drawings.

Figure 1 of the drawings represents the system described in the priorapplication. In this Figure l denotes a class B amplifier which isloaded with a resistance 2R.. In vthe output circuit of the class Bamplifier are located'the terminals 5 and 6 of an kimpedance invertingnetwork 1 whilst to the input terminals is connected a class C amplier 2whose threshold value is chosen by means of the bias Voltage in suchmanner, that this amplifier passes current only if the instantaneousvalue of the amplitude of the oscillations to be amplified surpasses apredetermined Value which is smaller than the unmodulated carrier waveamplitude. In the circuit which comprises the amplifier I, the loadvalue 2R and the terminals 5 and 6 is included the output impedance ofan impedance inverting network Il which has connected to its inputterminals a class C amplifier B whose threshold value is chosen by meansof the bias voltage in such manner, that the amplifier only passescurrent if the instantaneous value of the amplitude of the oscillationsto be amplified surpasses a pre-determined value which is greater thanthe unmodulated carrier wave amplitude. In the circuit which comprisesthe amplifier 2 and the terminals 8 and 9 ofthe network l is ineludedthe output impedance Vof an impedance network l0 which has connected toits input terminals a class C amplifier 3 whose threshold value has thesame value as the class C anplifier 4. The networks 1, I0 and Il arecalled impedance inverting because they possess the property that theinput impedance is inversely proportional to the impedance connectedbetween the output terminals, said property being reversible. The surgeimpedance Ro of the network 1 is R while those ofthe networks I0 and I Iamount to R/2. Sincev each of the networks 1, l0 and Il brings about aphase-displacement of the high-frequency alternating voltages suppliedto the input circuits of the class C amplifiers 2, 3, and 4 aredisplaced in phase by 90, 180, and 90 respectively, with respect to thevoltage supplied to the class B amplifier I.

If an unmodulated carrier oscillation is supplied to the ampliers, theclass B amplifier I is operative during the whole cycle and the class Camplifier 2 during only part of the cycle. During that part of the cyclein which the class C amplifier 2 is not conductive, the amplifier I isloaded with a resistance 2R. At the moment when the class C amplifier 2begins to supply energy, the instantaneous value of the voltage set upin the output circuit of the class B amplifier has attained its highestadmissible vene se that at this moment the amplifier I operates with'maximum efficiency. During that part of the cycle in which the class Camplifier 2 supplies energy, the load resistance of the class B arnpli-vfier I is smaller than 2R, owing to which an increase of the anodecurrent of the amplifying tubes becomes possible without any increase ofthe anode alternating voltage.

At the moment when the C amplifiers 3 and 4 begin to supply energy, theanode alternating voltage of the class C amplifier 2 has attained itshighest permissible value and the load resistance of the class B amplierI is equal to R whilst the load resistance of the class C amplifier 2also amounts to R. At this moment the amounts of energy furnished byeach of th'e two amplifiers 3 I and 2 are equal. When the instantaneousvalue of the oscillations to be amplied surpasses the threshold value ofthe class C amplifiers 3 and 4, the load of the amplifiers I and 2decreases. At the maximum voltage of the 100% modulated carrieroscillation the amplifiers 3 and 4 are fully loaded with the result thatthe load resistance of the amplifiers I andZ has decreased from thevalue R to the value 1/2R. Owing to an increase of the anode currentsAof the amplifiers I arid 2 the output energy of these ampliers hasincreased to double the value obtained at the moment when the thresholdvalue of the amplers 3 and 4 is surpassed. At the maximum voltage of a100% modulated carrier oscillation the amplifiers 3 and 4 are eachloaded with a resistance R/2 since th'e surge impedance Re of thenetworks I0 and I I amounts to R/ 2. At this moment each of the ampliersI, 2, 3, and 4 delivers an equal amount of energy and the elciency ofthe whole of the installation is equal to the maximum efciency of aclass B amplifier which furnishes the same output energy.

Figure 2 represents another system according to the prior applicationwhich is completely dual to the system according to Figure l and which'operates in a similar manner. In this system the class B amplifier I isconnected to a, load impedance R/ 2 through the intermediary of animpediance inverting network 'I which possesses the same property as thenetwork 1 in the system of Fig. 1. The class C amplifier 2 is connectedto the ends of this impedance. The surge impedance of the filter 1amounts to R. In the circuit which comprises the class B amplifier I andthe termin-als 8 and 9 of the network 'I is included the outputimpedance 'of an impedance inventing network II whose surge impedanceamounts to R/2 and to the input terminals of which is connected theclass C amplifier 4. In the circuit which comprises the class Camplifier 2 and the load resistance R/ 2 is included' the outputimpedance of an impedance inverting network I0 whose impedance amountsto R/ 2 and whose input terminals have a class C amplifier 3 connectedto them. The bias voltages and th'e mutual phase displacements of theoscillations supplied to the input circuits of the amplifiers I, 2, 3and 4 are the same as in the system according to Fig. 1. Also, theVariation of the load resistances of the different amplifiers as afunction of the modul-a.- tion is exactly similar to that occurring inthe previously described circuit arrangement.

It will be evident that when two of the amplifiers in Figures 1 and 2are earthed the two other' ampliers cannot be earthed unless they areinductively coupled with the circuit in which the load impedance isincluded.

'Ilhe 'present invention relates to a system which comprises, as doesthe above-described system, four amplifiers which successively supply,at different values of the high-frequency amplitude,Y energy t0. aconnnon load impedance and wherein all amplifiers can be earthed withontthe. use of inductive coupling.

According to th'e invention, thel class B amplifier I is connected tothe input terminals of an impedance inverting network to the outputterminals of which is connected one of the class C ampliiiers with ahigh threshold value 4, whilst the class C amplifier with the lowestthreshold value 2 is connected'to the input terminals of an impedanceinverting network to the output terminals of which is connected theother class C amplifier of vhigh threshold value 3, the latter amplifierbeing connected either thro-ugh the intermediary of the impedanceinverting network to a load impedance connected in parallel with theother class C amplifier of high threshold value 4 or through a loadimpedance tothe output termina-ls of an impedance inverting network tothe input terminals of which is connected the other class C amplifier ofhigh threshold value 4 whilst the surge impedance of the twofirst-mentioned networks amounts to double the surge im.- pedance of thelast-mentioned ne'twork and is equal to the load impedance or four timesas large as this impedance respectively and wherein th'e variousamplifiers are supplied with a phase such that the voltages supplied bythese amplifiers to the; load impedance are mutually in phase.

Figure 3 Vof the drawings represents one embodiment of the systemaccording to the invention. It comprises aclass B amplifier I which isconnected to the input terminals I2 and I3 of an impedance invertingnetwork I4 whose surge impedance amounts to R/ 2. The output terminalsI5 and I6 h'ave connected to them a 'class C ampliiier 4 whose thresholdvalue exceeds the unmodulated carrier wave amplitude. more, a class Camplifier 2 having a threshold value which islower than the unmodulatedcarrier wave amplitude is connected f, to the input terminals I'I, I8 ofan impedance inverting network IQ whose surge impedance' also. amountstoR/2. To the output terminals 20 and 2I of this network I9 is connectedanother class C, amplifier 3 whose th'reshol-d value exceeds theunmodula'ted carrier wave amplitude. This class C amplifier 3 isfurthermore connected' to the input terminals 22, 23 ofV an impedanceinverting network 24 whose surge impedance amounts to l/.LR and betweenthe output terminalsi25 and 28 of which is connected the common. loadimpedance 1/l't which is connected at the same time in parallel with,the class Cv amplifier 4.v

So long as the instantaneous value of the modulated rcarrier oscillationis located below Furtherthe threshold value of the class 'C amplifiers,solely the class B amplifier supplies energy to the load impedance %R.Since the load impedance is connected to the class B amplifier via animpedance inverting network I4 with a surge impedance R/Z, the latteramplifier is loaded during this period of time with an impedance $4122gli At the moment when the instantaneous value of the carrieroscillation is such that the class B amplifier is fully loaded and,consequently, the output Voltage of the class B amplifier has attainedthe maximum permissible value, the threshold value of the class Camplifier 2 is surpassed and in this case this amplier also suppliesenergy to the load impedance 143B. with the result that the impedance I2to I3 of the class B amplifier decreases and the anode current of thelatter increases without any increase of the output voltage. At themoment when the instantaneous value of the carrier oscillation is suchthat the class C amplifier is fully loaded and, consequently, also theoutput voltage of the class C amplier has attained the maximumpermissible value, the class B and class C amplifier I and 2,respectively, supply the same -amount of energy to the load impedancelAgR, which nvolves that the impedance between the terminals I2 and I3amounts to R and that the load impedance betwen the load terminals I'Iand I8 also amounts to R.

At this moment the threshold value of the class C amplifiers 3 and 4 issurpassed and the load impedance of the amplifiers I and 2 decreasesstill further owing to the fact that the amplifiers 3 and 4 also supplyenergy to the load impedance 1/8R. At the maximum instantaneous value ofa 100% modulated carrier oscillation, the amplifiers 3 and 4 are alsofully loaded and the load impedance of the amplifiers I and 2 betwen theterminals I2, I3 and I1, I8, respectively, has decreased to the valuelgR whilst also the amplifiers 3 and 4 are loaded in this case with animpedance 1/2R. All the four ampliers supply in this case the sameamount of energy to the load impedance 1A;R. Now, all amplifiers arefully loaded and operate with an efficiency which is equal to themaximum efciency of a class B amplifier which is about 63-67%.

A system equivalent to the system according to Figure 3 is representedin Figure 4. 'Ihe latter may be derived from the former by replacing thenetwork 24 with the load impedance 1/R between the input terminals 25,26 by the network 24 in Fig. 4 wherein a load impedance is connected inseries with the input terminals 22, 23.

Figure 5 represents a practical example of the system according toFigure 3. In Figure 5 the amplifiers I, 2, 3 and 4 are represented,comprising amplier tubes 4I, 42, 43 and 44 respectively, the anodes ofwhich are coupled to the respective filter circuits by means of couplingcondensers 45, 45, 41 and 48. Each impedance inverting network I4, 24and I 9 consists of a 1r lter the series impedance of which consists ofan inductance I4', 24 or I9', respectively, and the cross-impedance ofwhich consists of the parallel connectionof 'a condenserand aninductance. j j

The condenser and theinductance which are located between the inputterminals of the network I4 are denoted by 28 and 29, respectively. Thecondenser and the inductance which yare located between the outputterminals I5 and I6 of the network I4 and between the output terminals25, 26 of the network 24 are united to form a condenser 30 and aninductance 3|. In a similar manner the condenser 32 and the inductance33 form the cross-impedances of the neighboring networks 24 and I9. Thecrossimpedance connected between the terminals I1 and I 3 of the networkI 9 is represented by the condenser 34 and the inductance 35.v Each ofthe previously mentioned cross-impedances is tuned so as to form for thecarrier-wave frequency a capacitive reactance which is equal to theinductive reactance of the corresponding series impedances I4', 24' andI9', respectively. Although cross-impedances only consisting of acondenser would sufiice, it is advantageous, in View of the suppressionof the harmonics of the carrier wave, to utilize the above-mentionedparallel connection of a condenser and an inductance.

Figure 6 represents one embodiment of the system shown in Figure 4,which needs no further explanation. It is obvious that the currentssupplied by the amplifiers I, 2,3, and 4 to the load impedances l/BR(Figure 3) and 1/ZR respectively (Figure 4) must be in phase. Since thenetworks I4, 24 and I9 each bring about a phase displacement of theoscillations to be amplified must, consequently, be supplied to theseamplifiers via phase-displacing networks, in such manner that betweenthe oscillations supplied to the amplifiers I and 2 there exists a phasedisplacement of 90 whilst between the oscillations supplied to theamplifiers I and 4 or 3 and 4 respectively there exists a phasedisplacement of 90.

Although it is believed that my invention has now been made clear tothose versed in the art and that further illustration thereof isunnecessary, I have shown in Figure 7 a circuit arrangement such asshown diagrammatically in Figure 3. In Figure 7, I have shown how theseveral amplifiers are coupled by the networks without having any tubecathodes operating at high-radiofrequency potentials and thus withoutthe use of inductive couplings. The operation of the arrangement ofFigure 7 has been described above and need not be repeated here. It isnoted, however, that the blocking condensers BC are of sufficient sizeas to operate as short circuits for the radio-frequency potentials,whereby cirsuits 54, and 64 are seen to be operating in parallel and,therefore, can be constituted in practice by a single capacity and asingle Vinductance. The same remark applies to networks 'I4 and 84.

In place of shunt direct-current supply as shown in Fig. 7, I may useseries supply, if plate voltages for all of the tubes are to be thesame. In this case the direct-current blocking condensers BC areomitted.

What is claimedv is:

1. In a modulated wave amplifier, a load circuit, four electrondischarge tube amplifiers each having output electrodes and inputelectrodes, means coupling the output electrodes of one of said tubesdirectly to said load circuit, a phase inverting network coupling theoutput electrodes of a second one of said tubes to said load circuit, asecond phase inverting network coupling the output electrodes of a thirdone of said tubes to said load circuit, a third phase inverting networkcoupled to said second phase inverting network to couple the outputelectrodes of the fourth one of said tubes to said load, means forimpressing modulated wave energy on the input electrodes of each of saidtubes in such phase that the outputs of said tubes are cumulative insaid load, means for biassing the second of said tubes for class Boperation, and means for biassing the remainder of said tubes for classC operation.

2. An amplifier as recited in claim 1, wherein the remainder of saidtubes are so biassed that the fourth one of said tubes becomes operativebelow carrier amplitude and the said one tube and third tube becomeoperative for amplitude values above carrier amplitude.

3. An amplifier as recited in claim 1 wherein the input electrodes ofsaid second and third tubes are excited by modulated wave energy of afirst phase, the input electrodes of said one tube are excited bymodulated wave energy of a phase equal to said first phase +90 and theinput electrodes of said fourth tube are excited by modulated waveenergy of said first phase +270".

4. A system for amplifying a modulated oscillation, which is suitablefor the final stage of a transmitter and comprises four amplifiers ofwhich one amplifier operates as a class B amplifier and the otheramplifiers act as class C amplifiers with threshold values which are forone of these other amplifiers smaller and for the remaining otheramplifiers larger than the unmodulated carrier wave amplitudecomprising, means amplifiers, an impedance inverting network havinginput terminals coupled to saidclass B amplifier and having outputterminals coupled to one of the class C amplifiers of high thresholdvalue, a second impedance inverting network having input terminalscoupled to the said class C amplifier with the lowest threshold valueand having output terminals coupled to the other class C amplifier ofhigh threshold value, a third impedance inverting network, a couplingbetween said last named class C amplifier of high threshold value andsaid third network, a coupling between said third network and the otherclass C amplifier of high threshold value, and a load impedance in oneof said last two couplings, the surge impedance of the first and secondnetworks being about double the surge impedance of the third network andthe load impedance being adjusted to cause each of said amplifiers towork into equal effective output impedance at a load equal to twice theunmodulated carrier, the said various amplifiers being supplied withmodulated oscillations of such a phase that the voltages supplied by theamplifiers to the load impedances are.

of like phase.

5. A system for amplifying a modulated oscillation, which is suitablefor the final stage of a transmitter and comprises four amplifiers ofwhich one amplifier operates as a class B. amplifier and the otheramplifiersv act as class C amplifiers with threshold values which arefor one of these said other amplifiers smaller andl for the other` twoof said other amplifiers larger than the unmodulated oscillationamplitude comprising, means for impressing modulated oscillations forimpressing modulated oscillations on said onsaid amplifiers, animpedance inverting network having input terminals coupled to the classB amplier and having output terminals coupled to one of the said twoclass C amplifiers of high threshold value, a second impedance invertingnetwork having input terminals coupled to the class C amplifier with thelowest threshold value and having output terminals coupled to the otherclass C amplifier of high threshold value, a third impedance invertingnetwork, a coupling between said other and last named class C amplifierof high threshold value and said third network, a coupling between saidthird network and said one class C amplifier of high threshold value,and a load impedance in said last named coupling, the surge impedance ofthe first and second networks being about double the surge impedance ofthe third network and being substantially equal to four times as largeas the load impedance, the various ampliers being supplied withmodulated oscillations of a phase such that the voltages supplied by theamplifiers to the load impedance are of like phase.

6. A system for amplifying -a modulated oscillation, which is suitablefor the final stage of a transmitter and comprising four amplifiers ofwhich one amplifier operates as a class B amplifier and the otheramplifiers act as class C -amplifiers with threshold values which arefor one of these amplifiers smaller and for the other amplifiers largerthan the unmodulated carrier wave amplitude comprising, means forimpressing modulated oscillations on said amplifiers, an impedanceinverting network having input terminals coupled to the class Bamplifier and having output terminals coupled to one of the said class Camplifiers of high threshold value, a second impedance inverting networkhaving input terminais coupled to the said class C amplifier with thelowest threshold value and having output terminals coupled to the otherof said class C amplifiers of high thresholdl value, a third impedanceinverting network, a coupling between said last named other Class Camplifier of high threshold value and said third network, a loadimpedance in said1 last coupling, and a coupling between said thirdnetwork and said one class C amplifier of high threshold value, thesurge impedance of the first and second networks being about double thes-urge impedance of the network and being substantially equal to theload impedance, the various ampliers being supplied with a phase suchthat the voltages supplied by the amplifiers to the load impedance aremutually in phase.

7. A system as claimed in claim 4, wherein each of the impedanceinverting networks consists of a 1r filter whose series impedance isformed by an inductance and whose cross-impedance is formed by theparallel connection of an inductance and a condenser.

8. A system as recited in claim 4, wherein each of the impedanceinverting networks consists of a 1r filter whose series impedanceisformed by an inductance and Whose cross-impedance is formed by'the`parallel connection of an inductance and a condenser and whereinadjoining cross-impedances of adjacent networks have elements in common.

KLAAS- POS'IHUMUS.

